The present invention is a process for preparing a flowable slurry comprising mixing an alkaline material selected from the group consisting of chlorosilicon manufacturing byproducts, direct process residue gels, cement kiln dust and mixtures thereof, optionally clay, about 25 to 70 wt. % water and a process aid selected from the group consisting of sucrose, raffinose, lignin, methylglucopyranoside, lactose, fructose, sodium polyphosphate, trehalose and mixtures thereof to form the flowable slurry. The process is especially useful as an economical and an environmentally sound method for recycling alkaline raw materials for cement manufacture.
The preparation of portland cement is well known in the art. Portland cement is a hydraulic cement characterized by the ability to set and harden in water. Generally, portland cement is manufactured by mixing suitable raw materials with water, burning at suitable temperatures to effect clinker formation, and grinding the resulting clinkers to the fineness required for hardening by reaction with water. The burning operation generates a fine alkaline particulate byproduct called cement kiln dust that is difficult to recycle. Cement kiln dust typically comprises about 1-3 wt. % or more of the above product from the cement production operation.
The portland cement resulting from the above process consists mainly of tricalcium silicate and dicalcium silicate. These two materials are primarily derived from two raw materials: one rich in calcium such as limestone, chalk, marl, oyster or clam shells; and the other rich in silica such as clay, shale, sand or quartz.
Bunce, et. al in U.S. Pat. No.5,374,310 teach the use of a silicon containing product resulting from the hydrolysis of chlorosilicon by-product as materials for cement production. However, when such materials are added to a clay slurry for use in cement manufacture, a thick, non-flowable paste forms which prevents further downstream processing, especially when small amounts of water are used.
It would also be useful to use other alkaline materials such as other chlorosilicon manufacturing byproducts and cement kiln dust in the manufacture of cement. Unfortunately, these materials, too, suffer from the disadvantage that they form thick pastes that are difficult to manage unless large amounts of water are used.
The present inventors have now discovered that flowable slurries of such alkaline materials can be maintained by mixing them with certain process aids selected from the group consisting of sucrose, raffinose, lignin, methylglucopyranoside, lactose, fructose, sodium polyphosphate, trehalose and mixtures thereof.
The present invention is a process for preparing a flowable slurry comprising mixing:
25-70 wt. % water;
an alkaline material selected from the group consisting of chlorosilicon manufacturing byproducts, direct process residue gels, cement kiln dust and mixtures thereof;
optionally, clay; and
a process aid selected from the group consisting of sucrose, raffinose, lignin, methylglucopyranoside, lactose, fructose, sodium polyphosphate, trehalose and mixtures thereof
to form a flowable slurry.
The present invention is a process for preparing a flowable slurry comprising an alkaline material, a process aid, optionally clay and water. The process aid inhibits the formation of thick pastes that result when the alkaline material and, optionally, clay are mixed with water.
The alkaline materials useful herein comprise chlorosilicon manufacturing byproducts, direct process residue gels, cement kiln dust, and mixtures thereof.
The xe2x80x9cchlorosilicon manufacturing by-productsxe2x80x9d result from manufacturing operations in which chlorosilicons are produced. Typical of chlorosilicon manufacturing operations are the reaction of silicon metal with methyl chloride to form methyl chlorosilanes and the reaction of silicon metal with hydrogen chloride to produce chlorosilanes. Unfortunately, these operations produce a number of undesirable by-products and waste streams that require disposal.
These by-products and waste streams are often acidic and, thus, are often neutralized prior to use. An example of one such neutralization method is described in U.S. Pat. No. 5,000,934, in which a spent bed resulting from the reaction of halides with silicon metal is neutralized with a strong base and elevated temperature. This reference is incorporated herein by reference. The metal salts that are generated by processes such as this or other by-product disposal processes, as well as any other incidental metal salts generated by chlorosilicon production processes which will be known to those with ordinary skill are generally captured to avoid discharge to the environment and can be used in the process of the present invention.
A specific example of a by-product useful in the process of the present invention comprises that generated by the metal hydroxide precipitation during the purification of wastewater from chlorosilicon manufacturing.
The by-products resulting from these processes may comprise metal oxides, such as CaO, SiO2, Al2O3, Fe2O3, MgO, K2O, Na2O, SO3, TiO2, P2O5, SrO, Bao, and Mn3O4. While the actual composition is not critical to the invention, typical chemical compositions of the by-product from the example of the previous paragraph may comprise 0 to 30 wt. % CaO, 0 to 35 wt. % SiO2, 0 to 5 wt. % Al2O3, 0 to 4 wt. % Fe2O3, 0 to 8 wt. % MgO, 0 to 1 wt. % K2O, 0 to 0.5 wt. % Na2O, 0 to 1 wt. % SO3, 0 to 1 wt. % TiO2, 0 to 0.1 wt. % P2O5, 0 to 0.1 wt. % SrO, 0 to 0.1 wt. % BaO, 0 to 0.5 wt. % Mn3O4 and 0 to 30 wt. % water.
Another alkaline material useful in the present invention is direct process residue gel. Examples of this material are described in U.S. Pat. Nos. 4,408,030 and 5,374,310, which are incorporated herein by reference. These references describe the formation of gels from the by-products of the reaction of silicon metal with methyl chloride to produce chlorosilanes (the xe2x80x98direct processxe2x80x99). The gels are formed by the hydrolysis of a variety of these chlorosilion by-products including, for example, silanes, disilanes, disiloxanes, silane oligomers, siloxane oligomers, silphenylenes and silalkylenes. This hydrolysis occurs in an aqueous medium as described in the above patents, which may comprise additives such as HCl and/or CaCl2.
A typical composition of this gel comprises water, CaCl2, and the hydrolyzed products of the hydrolysis.
Also useful as alkaline materials in the present invention are other calcium rich materials, such as, cement kiln dust which, as described above, is a by-product generated during Portland cement manufacturing operations. Generally, Portland cements are manufactured by mixing suitable raw materials with water, burning at suitable temperatures to effect clinker formation, and grinding the resulting clinkers to the fineness required for hardening by reaction with water. The burning operation generates the fine alkaline particulate, cement kiln dust. While the actual composition is not critical to the invention, typical chemical compositions for cement kiln dust are in the range of about 15 to 55 wt. % CaO, 10 to 35 wt. % SiO2, 2 to 10 wt. % Al2O3, 1 to 6 wt. % Fe2O3, 1 to 11 wt. % MgO, 0.2 to 2 wt. % K2O, 0.05 to 1.0 wt. % Na2O, and 0.1 to 2 wt. % SO3.
The alkaline materials are typically added to the slurry of the present invention in amounts up to about 40 wt. % based on the total weight of the slurry when clay is present, and up to about 65% when clay is not part of the slurry. Often, the alkaline materials are used in amounts up to about 30 wt. % or, alternatively, about 1 to 30 wt. %, based on the total weight of the slurry when clay is included, and up to about 60 wt. % or, alternatively, 0.1 to 60 wt. %, when clay is not part of the slurry. Obviously, smaller amounts are also within the scope of the invention.
The process aids used in the present invention are selected from the group consisting of sucrose, raffinose, lignin, methylglucopyranoside, lactose, fructose, sodium polyphosphate, trehalose and mixtures thereof. These materials are all known in the art and readily commercially available. Preferably, the process is selected from the group consisting of sucrose, lignin, fructose and mixtures thereof. It should be noted that the use of precursors for these process aids as well as compositions containing these process aids are within the scope of this invention.
The amount of process aid used in the present invention depends on the other components used (i.e., the alkaline material used, the clay, and the water) as well as the quantities thereof. Generally, however, the process aid is used in an amount of from about 0.02 to about 10 wt. %, alternatively about 0.02 to about 5 wt. %, based on the weight of the slurry. Alternatively, the amount of process aid is within the range of about 0.05 to 1 wt. %.
The slurry of the present invention can also contain a clay. Clay is often included because clay slurries are a feedstock for cement manufacture and incorporating the alkaline material in clay slurry is a convenient method for introducing the alkaline material into the cement production process. While the type of clay utilized herein is not critical to the invention, typical examples include montmorillonite clay which comprises magnesium aluminum silicates suspended in water, often in suspensions of about 25-65% montmorillonite clay in water. Such clay suspensions are commercially available. Other clays, however, are also useful herein.
If used, the clay is usually added to the slurry of the present invention in amounts of about 1-65 wt. % clay solids, often above about 40 wt. %, alternatively above about 45wt. %, based on the total weight of the clay solids in the slurry.
Water is also a component of the slurry. Typically it is included in the slurry in amount of from about 25 to about 70 wt. %, alternatively from about 30 to about 55 wt. % based on the weight of the slurry. Values above 70% can be used, but the volume of water that needs to be removed in the calcining process becomes so large that it is economically prohibitive to boil it away due to fuel costs.
It should be noted that the above alkaline materials may make up all or only a portion of the calcium and silica requirements of Portland cement. As such, other alkaline, calcium and/or silica-containing materials can be added to the invention.
The order and method of mixing the components is not critical to the invention. For example, all of the components of the slurry can be mixed at the same time. Alternatively, the process aid can be mixed with a mixture of the water, alkaline material and optional clay, even after a thick paste has formed. Similarly, the alkaline material and optional clay can be mixed with aqueous solutions of the process aid.
These materials can be mixed by conventional mixing techniques and conventional mixing equipment such as, but not limited to, rod mills, pan mills pug mills, other single or multi-shaft mixers.
When the process aids of the invention are incorporated in slurries of the materials described herein, the slurries remain flowable and, thus, processable in cement manufacturing operations. Thus, not only are there significant savings in raw material costs in cement manufacturing, but this process eliminates costs associated with disposing cement kiln dust and it provides a means for recycling wastes associated with chlorosilane manufacturing operations.
The above slurries can be used in the manufacture of portland cement. This is generally accomplished by burning the slurries at suitable temperatures to effect clinker formation, and grinding the resulting clinkers to the fineness required for hardening when reacted with water.